In the materials known as liquid crystal compounds, a smectic liquid crystal or a cholesteric liquid crystal exhibiting a transparent liquid crystal texture causes a phenomenon that when the thin layer of the liquid crystal is locally heated and suddenly cooled, the portions are changed into an opaque liquid crystal texture scattering light. This phenomenon is known as a thermo-optical effect of liquid crystals. Various liquid crystal elements for writing information systems by applying a localized temperature change to the liquid crystal cell to opacity portions thereof by utilizing the above-described phenomenon have been proposed.
A material used for thermal writing liquid crystal elements must be stable to moisture, air, and light, and further must have an adequate phase transition temperature.
The term "adequate phase transition temperature" as used herein means a sufficiently wide temperature range including room temperature, under which thermal writing is possible at room temperature and where the storage temperature is not influenced, and it is desired that the liquid crystal is not transformed into other liquid crystal phase than a smectic phase or a cholesteric phase within that temperature range.
A Schiff's base series liquid crystal material shown by the following formula ##STR5## (wherein R represents an alkyl group), which is generally used as a light valve element used for a projection type display system tends to be hydrolyzed whereby the material has a short life time and is unreliable due to low humidity resistance.
Also, as chemically stable compounds, a compound shown by the formula ##STR6## (wherein R represents an alkyl group) is reported but this compound has an insufficient liquid crystal temperature range.
Also, various compounds are known as liquid crystal compounds and these compounds are called nematic liquid crystals. These nematic liquid crystals are the main compound or composition being used in liquid crystal displays at the present time. However, one of the disadvantages of these compounds is that the response speed is slow and a response speed of on the order of a few milliseconds only is obtained. Hence it has been said that the use of such a compound has approached the limit for large-scale displays.
To correct the above-described disadvantage of conventional liquid crystal display elements, the use of a bistable liquid crystal is proposed in JP-A-56-107216 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). A bistable liquid crystal is called a ferroelectric liquid crystal and it is being watched with keen interest since a fast response and memory are possible by using this type of liquid crystal. In particular, practiced uses of this type of liquid crystal have been actively investigated and the development of a ferroelectric liquid crystal material for practical use has become urgently needed.
In general, a ferroelectric liquid crystal is a compound having an optically active site. The characteristics thereof are achieved by a series of smectic phases having a molecular orientation so that the long axis of the molecule is tilted from the normal direction of the layer. Of these smectic phases, a chiral-smectic C (hereinafter, referred to as Sc*) phase is considered the most promising for practical use due to the relatively low voltage requirements thereof for operation.
Since a ferroelectric liquid crystal has a very fast response speed due to the spontaneous polarization thereof and also can realize a bistable state with memory capabilities and further has excellent view field, the ferroelectric liquid crystal is suitable as a material for a large capacity and large picture size display.
Examples of such ferroelectric liquid crystals include 4-(4'-n-decyloxybenzylideneamino)cinnamic acid 2-methylbutyl ester (hereinafter referred to as DOBAMBC) as described in J. Physique, 36, L-69 (1975).
DOBAMBC has a disadvantage in terms of chemical stability since it contains a Schiff's base in the structure. Thus, various physically and chemically stable compounds have been investigated as materials for ferroelectric liquid crystals. At present, 2-methylbutyl 4-(4'-n-alkyloxybenzoyloxy)benzoate (hereinafter, referred to as CN) and other similar ester series compounds have been mainly investigated. However, since CN and other similar ester series compounds do not exhibit a Sc* phase or, even when these compounds show the Sc* phase, the temperature range within which the Sc* phase is exhibited is narrow. Also these compounds are monotropic liquid crystal each exhibiting different phase series between heating the liquid crystal and cooling the liquid crystal. Thus, few compounds are capable of being practically used as described in Liquid Crystals and Ordered Fluids, 4 (1984).
On the other hand, the introduction of a heterocyclic ring (e.g., a pyridine ring, a pyrimidine ring, etc.) or the introduction of a cyclohexane ring into a molecule of such compounds is generally used to shift its liquid crystal phase temperature range to a temperature range including room temperature. However, the synthesis method thereof is complicated.
In particular, since the introduction of a cyclohexane ring for improving the liquid crystal property has the problems that the smectic property is lost by the introduction of the cyclohexane ring and the synthesis requires the cis isomer of 1,4-di-substituted cyclohexane and the trans isomer thereof to be separated from a mixture thereof, sufficient investigation of these compounds has not been made.
Various ferroelectric liquid crystal compounds containing a hetero atom are known and examples include ferroelectric liquid crystal compounds containing a nitrogen atom, compounds containing a nitrogen atom in a heterocyclic ring such as a pyrimidine ring as described above (JP-A-61-22072, JP-A-61-24576, and JP-A-61-129170) and compounds containing a Schiff's base as a linking group, such as DOBAMBC, are known.
Of the compounds containing a nitrogen atom as a linking group, compounds containing a Schiff's base have chemical stability problems as described above. Also, the compounds containing an amide bond are not effective since they have a high melting point and tend not to show a liquid crystal property.
Examples of compounds containing a nitrogen atom in the form of an amine such as those containing an aromatic secondary amine are disclosed in JP-A-63-2961 and JP-A-2-53756. However, the temperature ranges of these compounds showing the liquid crystal property are all narrow. Thus, these compounds have not yet been practically used.